staging: lustre: lov: ld_target could be NULL

lov_device::ld_target[ost_idx] could be NULL if the OST target is
not filled in lov_device::ld_lov::lov_tgt_desc[ost_idx] yet.

Signed-off-by: Bobi Jam <bobijam.xu@intel.com>
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-8018
Reviewed-on: http://review.whamcloud.com/21411


Reviewed-by: Jinshan Xiong <jinshan.xiong@intel.com>
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>
Signed-off-by: James Simmons <jsimmons@infradead.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>